Visionary Michael Mann didn't manage it very often to bring his favored version of a theatrical movie into cinemas. So it was almost common that later, for home theaters, a newly cut Director's Edition was released, which often differed greatly from the original theatrical version. Looking at the respective Director's Cuts you can see with which theatrical versions Mann was especially not satisfied with, for example Manhunter, The Last of the Mohicans and Ali.
It's apparently completely different with theatrical cuts of movies the director sees as nearly perfect, for example "Thief". A Director's Cut edition was released for this movie, too; but it only altered very little content compared to the original version (anything else would have been a crime with this excellent film, anyways).

When Mann brought together acting stars Al Pacino and Robert De Niro in 1995 together with a further perfect Cast in "Heat, it was a long dream come true. The same subject, which is based on some real events resp. characters he already picturized six years earlier in "L.A. Takedown", unfortunately only on TV-level at the time with corresponding actors, compelled to big compromises (i.e. curtailments) on the details of the story.
With "Heat" it was different. Here, the adept director could draw on unlimited resources and already made the theatrical cut his final cut, which features hefty 170 minutes of runtime. So the movie was never released in the form of a Director's Cut or anything alike, because the Theatrical Version already was just that.
So it was just the more interesting when, at the announcement of the Blu-Ray Disc, the information New content changes supervised by Director Michael Mann on the back cover was revealed, particularly since there was not really a big runtime difference to be seen:

Well: There actually isn't one - only three seconds make the difference between the two versions that are at hand. Just like back then with "Thief", Mann only changed very little resp. in this case almost no content. Only two small lines in two different scenes had to go, while one was replaced with an alternate shot and the other was simply removed from the sound track. The removed line by Diane Verona is a little fustian and pretty badly spoken, which is probably why it had to be removed according to Mann; Pacino's removed commentary at his burst towards Hank Azaria is probably supposed to enhance the priceless expression on the latter one's face, this short, silent moment actually does make a bigger impression than the additional comment by Pacino in the theatrical cut.

There actually aren't any more changes beside these two small ones; assumptions that pop up again and again in several film- & DVD- (resp. Blu-Ray-) forums, that there were e.g. completely different shots used in the restaurant scene or additional shots of policemen were implemented just before the shooting in front of the bank proved themselves simply wrong.
Next to the hardly existing contentual differences, the Blu-Ray version unfortunately doesn't even improve on the quality side, which film fans might have wished for. Even considering the age of the movie and the fact that, comparing the DVD and the Blu-Ray, you can of course see an obvious difference in Image & Sound, a little more effort could have been made transferring. Still, this silver disc naturally belongs in any demanding filmfan's collection, even though you might consider a low price offering.

Runtime designations of the described scenes relate to the Theatrical cut.
The rest of the runtime differences results from the different runspeeds PAL-NTSC.

The uncut theatrical version, represented by the German FSK 16 DVD by Warner Home Video has been compared to the US Blu-Ray (also by Warner).

Runtime of the theatrical cut:

2:43:30 (2:39:05 w/o credits)

Runtime of the Blu-Ray:

2:50:27 (2:45:50 w/o credits)

1 alternate shot
1 changed line of dialouge

1:01:09In the theatrical cut, the shot keeps showing Justine, the Blu-Ray version cuts to Vincent who looks pretty bored while his wife is having a monolouge. Here, the first part of her line "You sift through the detritus..." was left out, the shot of Vincent in the BD version is therefore a little shorter than the successive scene with Justine in the TC.
3 Sec.

Theatrical Cut:

US Blu-Ray:

1:15:28In the Blu-Ray version, the small line "Ferocious, aren't I?" was removed from the sound track.
no time difference